Dead Cylinder - No explanation

Hi,

The number 8 cylinder header pipe on my SBC is cold. All the other pipes get hot, but the number 8. I've checked for an intake blockage. i've checked for a bad cam lobe by removing the valve cover and checking the rockers. I've checked for no spark. I checked the distributor cap for a bad terminal. I checked for compression in the cylinder. There are no blockages on the intake side. The rockers seem OK. There is spark. The distributor cap appears fine. The compression is 190.

The only thing I haven't checked for yet is something blocking the exhaust port. I don't think there is a blockage there either because I'm getting a muffler backfire on the passenger side that I think is caused by the unburnt fuel from the #8 cylinder in the accumulating in the muffler and eventually going off. So if there's gas in the muffler, there's no blockage in the exhaust port.

Besides the blocked exhaust port (any ideas what could block it?), what else could be happening? I'm stumped.

Help!

Thanks!

 

did you put in a NEW spark plug? and double check the order of the plug wires?

I've seen guys fight this problem for days, I put in a new set of plugs and it purrs like a kitten....no shit

 

Does the plug come out wet or dry, run it with the plug wire off, does the plug come out wet or dry?

 

I swapped the #6 plug wire with the #8 plug wire and changed the plug in the #8. I also pulled the new plug and watched it spark outside the cylinder. So I feel like I've eliminated the plugs and wires.

 

Ok, but still, it would make me feel better if you'd spend $2 and put a NEW plug in the dead hole....if it hadn't happened to me before I wouldn't suggest it.

 

The plug comes out dry with the wire on. I'll have to check it with the wire off. I'll have to wait until tomorrow for that, I have the carb off from checking the intake for a blockage.

The compression gauge fitting came out with a little bit of gas on it. As a matter of fact, it came out of the #4 cylinder with the same bit of gas on it.

 

I'll give that a try too.

 

first, this thing isn't fuel injected, is it?

 

No. Holley double pumper. Weiand single plane intake.

 

Air leak?

 

Had an insulator break way up high, and intermittently slip down the electrode and shroud the gap. Drove me crazy until I isolated the cylinder and put an old plug in the hole.

 

I'm not sure what you mean.

 

I have to try a new plug, but I've had 2 plugs and 2 wires on the cylinder and it didn't change anything. I hope it's as simple as a bad plug though.

I even thought the cylinder might be getting so loaded with fuel the plug wasn't able to spark, but then why wouldn't the #7 be just as bad? Besides, my float levels are ridiculously low.

 

I had the same problem on an sbc ... It was the plug....

 

I agree with koppak you have a vacume leak on that one cylinder only, are there any vacume hoses that run off the intake on the runner to that cylinder? If so chase that hose looking for a leak. If not spray some carb cleaner at the intake gasket for that cylinder with the engine running I think you will find it will come to life when sprayed and then die again after a few seconds. Being a master auto tech of 23 years experiance I have seen this many times. Good luck.

 

There are no vacuum lines on the motor at all.

I'll give the carb cleaner a try tomorrow too.

Thanks for the help, everyone.

 

I've seen guys that know better make this mistake. That's my best guess.

 

The plug wires were never touched - until I swapped the #8 for the #6 after finding the problem.

I switched them back after I pulled the distributor cap to check that for problems.

Surprisingly, the engine runs pretty well on 7 cylinders. I only noticed the problem when I was adjusting the idle screws and I was getting cooked on the driver's side of the motor reaching in for the screws and the passenger side was so much cooler. It didn't seem right and I ended up spitting on the headers pipes and the 2,4 and 6 sizzled and the 8 was cold.

If it weren't for that, I probably still wouldn't have noticed it.

 

does that tube get warm when you hold it revved up, or if you go for a short drive?

 

Look for a grey spot on the wires. You can get a good spark outside the motor but leak out the wire under compression. One of the things that makes plug gap important is that as the cylinder pressure rises, it becomes harder for the spark to jump the gap. If the wires are questionable at all, change 'em.

 

If your motor is a "closed" PCV system, it could be a bad intake gasket.Sometimes the motor can backfire and blow JUST the lower side of the intake gasket out into the lifter galley allowing that intake port to draw air from the crankcase....this is not as obvious as a normal vacuum leak as the closed system minimalizes the effect- unlike a regular vaccum leak.
This would account for the lean plug reading and lack of fuel fouling [thus a cold /low fuel intake situation in that cylinder only....
Id uncork the breathers/PCV valve and see if the idle speeds up radically.
Hope you find it -
It ~~is~~ frustrating sometimes.....

 

This is what leads me to suspect the intake gasket- a lot of times a motor is set up with improper crankcase venting and it will invariably blow a gasket easier- whether it be an oil seal or intake gasket it WILL happen.....
It also kinda explains readings with the proper amount of compression in that hole -unlike a valve leak.....

 

It doesn't get warm enough that you can't touch it. I can't drive it. No registration yet. Not to mention that the radiator they used for racing is useless for street driving. It's got to go.

Choprods, I'm going to check for a leak at the gasket during the day tomorrow. I just came in from putting the carb back on the motor. So it's ready to go.

Scotty, The plug wires are MSD wires. I guess they should be a blue color. They're kind of grey now. They're pretty damn nasty looking. They're probably up for getting replaced.

Thanks for the help.

 

Just a thought, My brother-in-law had a sbc that kept burning the #8 cylinder piston. They checked everything, and changed everything with no luck. After the 3rd motor, they discovered that the PCV Valve was positioned above the #8 cylinder. The baffel plate in the valve cover he was using was missing and the PVC was sucking the oil up out of that cylinder. Replaced the baffel under the pvc valve, and the problem went away!
So where is the position of your pvc valve? If it is above the #8 cylinder, it could be pulling enough oil past the rings to cool the combustion temps, which could be causing your problem.
Gene

 

I've got one breather over the #1 and one over the #2. Each breather has a hose going to a bung on the each header.

 

you say you checked the rocker arms...did you crank the motor over and check to make sure they were opening and closing correctly





Fred

 

I've always heard that this is common problem w/SBC's. It seems that the #8 cylinder has a cooling system issue that causes the piston rings to seize up. The result is bad compression, but it may be subtle enough as to not be detectable with a cranking compression check.

 

It could be a leaking intake gasket or a weak/broken valve spring.

 

You could also have a worn valve guide causing this problem.Also you never said ,did the motor use to run correctly or has it always had this problem?you could spray some carb clean in the breather hole to see if idle smooth's out.If it does it's either gonna be a guide or intake gasket.GREGG

 

Racefab, the rockers seem to be working properly.

BigMikeC, I hope that's not the problem.

Larry T, I'm checking for that today.

Gregg Pellicer, the motor runs great even with the dead cylinder. I didn't even notice it except that the passenger side was cooler than the driver's side (where my arm was getting cooked), when I had my arm over the headers when I was reaching in to adjust the idle screws on the carb yesterday. I figured out one of the header tubes was cold using the high tech "spitting on them" method.

And here we are.

Thanks again everyone. I'll be able to check on the car in a couple hours or so and I'll post back.